An aspect of a switching power supply of the prior art is a chopper-type step-down switching power supply which steps down a DC voltage applied to an input terminal and outputs the voltage from an output terminal (for example, see Japanese Patent No. 3691500).
The switching power supply configured thus according to the prior art will be described below in accordance with the accompanying drawing.
FIG. 10 is a circuit diagram showing the configuration of the switching power supply of the prior art. The circuit configuration of FIG. 10 is made up of a switching element 101, a switching element control circuit block 102, a capacitor 103 for generating the power supply voltage of the switching element control circuit block 102, a converter circuit made up of a diode 104, a coil 105, and an output capacitor 106 for smoothing voltage, an output voltage detecting control circuit block 107, a capacitor 108 for generating the power supply voltage of the output voltage detecting control circuit block 107, and resistors 109 and 110 for detecting an output voltage.
When a DRAIN terminal of the switching element 101 is fed with a voltage (a voltage obtained by rectifying a commercial AC power with a rectifier such as a diode bridge and smoothing the power through a smoothing capacitor and the like, or a DC voltage) from an input terminal IN, a regulator in the switching element control circuit block 102 supplies a current to the capacitor 103 and the power supply voltage is generated for the operation of the switching element control circuit block 102. After that, when the voltage generated by the capacitor 103 reaches at least the starting voltage of the switching element control circuit block 102, the switching element control circuit block 102 starts the on-off control of the switching element 101.
When the switching element 101 starts an on/off operation, power is supplied to the converter circuit made up of the diode 104, the coil 105, and the output capacitor 106 and the output voltage of an output terminal OUT increases. The output voltage is divided by the resistors 109 and 110 and a signal V01 of the divided voltage is inputted to the output voltage detecting control circuit block 107. When the output voltage increases, a regulator in the output voltage detecting control circuit block 107 supplies a current to the capacitor 108 and the power supply voltage is generated for the operation of the output voltage detecting control circuit block 107. After that, when the voltage generated by the capacitor 108 reaches at least the starting voltage of the output voltage detecting control circuit block 107, the output voltage detecting control circuit block 107 becomes operable and starts detection control on the output voltage of the output terminal OUT.
After that, when the output voltage of the output terminal OUT is a specified value or higher, the output voltage detecting control circuit block 107 outputs a feedback current signal IFB, which reduces the peak current value of the switching element 101, to the switching element control circuit block 102. The switching element control circuit block 102 controls the on-off of the switching element 101 in response to the feedback current signal IFB, thereby achieving current mode PWM control according to an output load condition. When the output is under light-load conditions, the number of feedback current signals IFB from the output voltage detecting control circuit block 107 to the switching element control circuit block 102 further increases and the switching element control circuit block 102 decides that the output is under light-load conditions and suspends or stops the switching element 101. When the output voltage of the output terminal OUT decreases, the on-off control of the switching element 101 is restarted.
By performing switching control thus according to an output load, power saving can be achieved under light-load conditions.
In the foregoing chopper-type step-down switching power supply, the switching element 101 and the switching element control circuit block 102 are disposed on the high side of an input voltage applied to the input terminal IN and the output voltage detecting control circuit block 107 is disposed on the low side of the input voltage applied to the input terminal IN. In this switching power supply of the prior art, the output voltage detecting control circuit block 107 does not operate unless the output voltage of the output terminal OUT reaches at least a power supply voltage necessary for the output voltage detecting control circuit block 107 at power-on and so on. Thus the feedback current signal IFB cannot be transmitted to the switching element control circuit block 102, so that feedback control cannot be normally performed.
In a period during which feedback control cannot be normally performed, the detected peak current value of the switching element 101 is fixed at the upper limit value which is determined beforehand by the switching element control circuit block 102. Further, there is always a delay time between when the switching element control circuit block 102 detects the peak current value and when the switching element 101 is turned off. In this case, when energy is hardly consumed in the off period of the switching element 101 because of no load on the output terminal OUT, a current passing through the switching element 101 has a continuous waveform at power-on. For example, even when the switching element control circuit block 102 detects the peak current value at the turn-on of the switching element 101, the value of current passing through the switching element 101 gradually increases because of the delay time.
Consequently, the on-duty width of the switching element 101 is kept at the minimum (will be referred to as the minimum pulse width), the control of the peak current value is kept disabled, and the value of current passing through the switching element 101 increases in the period during which feedback control is not normally performed, so that the switching element 101 may be broken.
Further, when the output terminal OUT is overloaded during a steady-state operation and the output voltage decreases, the output voltage detecting control circuit block 107 is not operated, so that the switching element control circuit block 102 cannot normally perform feedback control as has been discussed.